1. Field of The Invention
The present invention generally relates to image compression techniques, and more particularly to a digital halftoning method in which a gray-scale image is compressed into a halftone image with a limited number of gray levels. The digital halftoning is an important technique that is applied to processed images or images being processed when visualizing such images by using an output device, such as a display or a printer.
Further, the present invention relates to a digital halftoning apparatus which carries out digital halftoning processing. The present invention relates to a computer readable medium storing program code instructions which cause a processor to execute digital halftoning processing.
2. Description of The Related Art
Digital halftoning used in imaging systems is known as a technique for generating the pixel patterns with a limited number of gray levels or colors. For example, two gray levels are used to generate a bilevel image, eight colors are used to generate a RGB color image, and 16 colors are used to generate a CMYK color image. As the approach for digital halftoning of this type, various conventional methods, such as the thresholding method, the dithering method or the error diffusion method, have been developed.
Recently, the ability of data processing and computation in microprocessors and digital signal processors has remarkably been increased, and attention is being given to model-based digital halftoning.
As disclosed in Japanese Laid-Open Patent Application No.5-91331, a few digital halftoning techniques are known. These techniques may be categorized into a first scheme for digital halftoning based on a human visual model, and a second scheme for digital halftoning based on a printer characteristic model. The human visual model used by the first scheme is to represent a digital halftone image with improvement of the quality of the image from the viewpoint of the human vision. The printer characteristic model used by the second scheme is to represent a digital halftone image with improvement of the quality of the image from the viewpoint of the printer output characteristics.
Generally, the human visual model depends upon the resolution of the digital image and upon the eye-to-paper distance between the human eye and the paper. The human visual model may be implemented by using a digital finite impulse response (FIR) filter.
Further, the printer characteristic model is defined with a dot radius of the printer, and it is provided by using a lookup table in which pixel values of the respective pixels corresponding to the digital image are locally arranged so as to be in conformity with the printer output characteristics. In particular, the least square model-based (LSMB) method, which is known as one of the model-based digital halftoning techniques, is characterized by the computation of an optimum halftone image so as to minimize the human visual errors between the original gray-scale image and the printout halftone image.
The advantage of the model-based digital halftoning is that the model parameters are adjusted based on the image and output device characteristics, and the adaptive control of an output image is provided. It is known that the quality of halftone images created by the model-based digital halftoning can be higher than the quality of images created by the screening or the error diffusion method. The model-based digital halftoning will improve the smoothness of the image texture and remarkably reduce the undesired effects, such as the so-called xe2x80x9cwormxe2x80x9d.
On the other hand, some approaches for improvement of the quality of images created by the screening or the error diffusion method have been proposed. In an improved error diffusion method, the smoothness of the image texture is increased by setting, at random, the filter weight coefficients, and the edge sharpening is attained by the modulation of the threshold values.
In the above-mentioned LSMB method, the computation of the optimum halftone image for minimizing the human visual errors requires a fair amount of computations on the microprocessor or the digital signal processor.
In the digital halftoning, the information contained in the original image will be partially lost because the number of gray levels or colors is reduced from that of the original image. However, it is desirable to retain, from the viewpoint of the human vision, the appearance of the original gray-scale image in the resulting halftone image. For this purpose, an improved image coding method that does not cause the loss of the human visual information in the resulting halftone image is demanded. Preferably, the improved image coding method is capable of not only retaining the image features (for example, the image smoothness) but also providing improvement of the image quality from the viewpoint of the human vision.
For example, when the image texture is complicated or there is an abrupt change in the pixel value in the neighborhood of a particular pixel in the original image, the edge sharpening should be provided in the halftone image. This measure is often taken for character or graphics regions of the original image. When the original image contains a smooth region where the pixel value scarcely changes, the moire or other synthetic patterns should be avoided from appearing in the halftone image.
However, in the above-mentioned model-based digital halftoning methods, it is difficult to provide the adaptive control of the quality of the output image even if only one of either the smoothness or the sharpness can be improved. Specifically, it is difficult for the above LSMB method to improve both the smoothness and the sharpness. The use of the FIR filter provides improvement of the smoothness or the sharpness of the entire output image. If the FIR filter is configured to improve the smoothness, the sharpness of the output image will be lost. In a case of a mixed image in which the characters and the graphics coexist, the characters of the output image will be difficult to read. On the other hand, if the FIR filter is configured to improve the sharpness, the moire or other synthetic patterns will appear in the smooth regions of the output image.
Similar problems arise with the other conventional methods. For example, in the improved error diffusion method, the smoothness of the image is increased by setting, at random, the filter weight coefficients, and the edge sharpening is attained by the modulation of the threshold values. However, the method of this type does not use a quantitative parameter model that directly affects the quality of the output image, and it is difficult to control both the smoothness and the sharpness of the output image by adjusting a small number of quantitative parameters.
An object of the present invention is to provide an improved digital halftoning method and apparatus in which the above-mentioned problems are eliminated.
Another object of the present invention is to provide a model-based digital halftoning method that provides the adaptive control of the human visual quality of the output image by using a visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness, without excessively increasing the amount of computations needed to achieve the digital halftoning.
Another object of the present invention is to provide a model-based digital halftoning apparatus that provides the adaptive control of the human visual quality of the output image by using a visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness, without excessively increasing the amount of computations needed to achieve the digital halftoning.
Another object of the present invention is to provide a computer readable medium storing program code instructions for causing the computer to execute a model-based digital halftoning processing that provides the adaptive control of the human visual quality of the output image by using a visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness, without excessively increasing the amount-of computations needed to achieve the digital halftoning.
The amount of memory available to the digital halftoning hardware is limited. If the amount of computations needed to achieve the digital halftoning exceeds the amount of memory available, it is impossible for the hardware to carry out the digital halftoning. Still another object of the present invention is to provide a model-based digital halftoning method and apparatus that effectively reduces the amount of computations needed for the digital halftoning.
The above-mentioned objects of the present invention are achieved by a digital halftoning method for compressing a gray-scale image into a halftone image, the halftone image having pixels with a decreased number of gray levels, which includes the steps of: providing a visual-quality control filter based on computation of a quantity of local features of neighboring pixels for each of respective pixels of the gray-scale image, the visual-quality control filter adaptively controlling visual characteristics for each of respective pixels of an image input to the filter; generating an initial halftone image from the gray-scale image; processing the gray-scale image through the visual-quality control filter to output a processed gray-scale image with the controlled visual characteristics; processing the initial halftone image through the visual-quality control filter to output a processed initial halftone image with the controlled visual characteristics; computing a sum of squares of local visual-quality errors between the processed gray-scale image and a processed halftone image, wherein the computation is started by taking the processed initial halftone image as a first processed halftone image; and generating an output halftone image from the initial halftone image through a repeated computation such that the sum of the squares of the local visual-quality errors obtained for the gray-scale image and the initial halftone image is reduced to a smaller value obtained for the gray-scale image and the output halftone image.
The above-mentioned objects of the present invention are achieved by a digital halftoning method for compressing a gray-scale image into a halftone image, the halftone image having pixels with a decreased number of gray levels, which includes the steps of: partitioning the gray-scale image into blocks of pixels of partial overlapping image portions; providing a visual-quality control filter based on computation of a quantity of local features of neighboring pixels for each of the respective blocks of the gray-scale image; processing each of the respective blocks of the gray-scale image through the visual-quality control filter so as to output processed partial overlapping image portions each with the controlled visual quality; generating an initial halftone image portion from each of the respective blocks of the gray-scale image; processing each initial halftone image portion through the visual-quality control filter so as to output processed initial halftone image portions each with the controlled visual quality; computing a sum of squares of local visual-quality errors between the processed gray-scale image and a processed halftone image portion for each block of the gray-scale image, wherein the computation is started by taking the processed initial halftone image portion as a first processed halftone image; generating an output halftone image portion from the initial halftone image portion through a repeated computation, for each block of the gray-scale image, such that the sum of the sequares of the local visual-quality errors obtained for the gray-scale image block and the initial halftone image protion is reduced to a smaller value obtained for the gray-scale image block and the output halftone image portion; and generating an output halftone image by composition of the output halftone image portions obtained for all of the blocks of the gray-scale image.
The above-mentioned objects of the present invention are achieved by a digital halftoning apparatus for compressing a gray-scale image into a halftone image, the halftone image having pixels with a decreased number of gray levels, which includes: an image memory which stores a gray-scale image and a halftone image; an initial halftone image computation unit which generates an initial halftone image from the gray-scale image, and stores the initial halftone image in the image memory; an image feature computation unit which provides computation of a quantity of local features of neighboring pixels for each of respective pixels of the gray-scale image; a visual-quality control filter unit which provides a visual-quality control filter based on the computation of the quantities of local features of the neighboring pixels for the respective pixels of the gray-scale image, the visual-quality control filter adaptively controlling visual characteristics for each of respective pixels of an image input to the filter, so the the controlled visual characteristics of the processed input image are stored in the image memory; and an optimum halftone image computation unit which computes a sum of squares of local visual-quality errors between a processed gray-scale image output by the filter and a processed halftone image output by the filter, the computation being started by taking a processed initial halftone image output by the filter, as a first processed halftone image, and generates an output halftone image from the initial halftone image through a repeated computation such that the sum of the squares of the local visual-quality errors obtained for the gray-scale image and the initial halftone image is reduced to a smaller value obtained for the gray-scale image and the output halftone image.
The above-mentioned objects of the present invention are achieved by a computer readable medium storing program code instructions for causing a processor to execute a digital halftoning processing in which a gray-scale image is compressed into a halftone image, the halftone image having pixels with a decreased number of gray levels, the computer readable medium including: a first program code device which causes the processor to provide computation of a quantity of local features of neighboring pixels for each of respective pixels of the gray-scale image; a second program code device which causes the processor to provide a visual-quality control filter based on the computation of the quantities of local features of the neighboring pixels for the respective pixels of the gray-scale image, the visual-quality control filter adaptively controlling visual characteristics for each of respective pixels of an image input to the filter; a third program code device which causes the processor to generate an initial halftone image from the gray-scale image; a fourth program code device which causes the processor to process the gray-scale image through the visual-quality control filter to output a processed gray-scale image with the controlled visual characteristics; a fifth program code device which causes the processor to process processing the initial halftone image through the visual-quality control filter to output a processed initial halftone image with the controlled visual characteristics; a sixth program code device which causes the processor to compute a sum of squares of local visual-quality errors between a processed gray-scale image output by the filter and a processed halftone image output by the filter, the computation being started by taking a processed initial halftone image output by the filter, as a first processed halftone image; and a seventh program code device which causes the processor to generate an output halftone image from the initial halftone image through a repeated computation such that the sum of the squares of the local visual-quality errors obtained for the gray-scale image and the initial halftone image is reduced to a smaller value obtained for the gray-scale image and the output halftone image.
In the digital halftoning method and apparatus of the present invention, the sum of the squares of the local visual-quality errors between the processed gray-scale image output by the visual-quality control filter and the processed halftone image outptu by the visual-quality control filter, and the output halftone image is generated from the initial halftone image through a repeated computation such that the sum of the squares of the local visual-quality errors obtained for the gray-scale image and the initial halftone image is reduced to a smaller value. The digital halftoning method and apparatus of one preferred embodiment of the invention is effective in providing the adaptive control of the human visual quality of the output haltone image by using the visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness. It is possible to prevent the amount of computations needed for the digital halftoning from being excessively increased.
Further, in the digital halftoning method and apparatus of the present invention, the gray-scale image is partitioned into blocks of pixels of partial overlapping image portions, and the output halftone image is generated by composition of output halftone image portions obtained for all of the blocks of the gray-scale image. The digital halftoning method and apparatus of one preferred embodiment of the invention is effective in reducing the amount of computations needed for the digital halftoning.